1. Field of the Invention
The present invention relates to a method for connecting a printed circuit board with a conductive part of a housing, for suppressing radiant noise from electronic equipment, and to an electronic instrument having the connection structure according to the connecting method.
2. Related Background Art
For suppressing the transmission noise and radiant noise of electronic equipment, it is necessary to decrease potential variation of the ground provided in the printed circuit board. This is because the radiant noise and transmission noise is suppressed by stabilizing the potential of the ground and thereby stopping generation of unwanted current.
The methods for connecting the printed circuit board with the conductive part of the housing in which the printed circuit board is mounted are examples of methods for stabilizing the ground potential of the printed circuit board. In such methods a normal connecting method is as shown by the potion of connection to the external ground in "Jissen noise teigen giho (Practical noise decreasing techniques)," (Jatech Shuppan), p. 342, FIG. 11 to FIG. 16, wherein the connection with the conductive part of the housing as the external ground with stable voltage change is such that the connecting portion has low impedance. In this case, however, the electric current flowing in the printed circuit board and the conductive part of the housing generates a standing wave at a specific frequency determined by the impedance of the circuit board to the conductive part of the housing, which accounts for the radiant noise.
FIG. 29 is a partial side view of a connecting portion of printed circuit board with the housing in a conventional example, wherein reference numeral 111 designates the printed circuit board, 112 the conductive part of the housing, and 113 a connecting screw of the printed circuit board.
For example, as shown in FIG. 29, there is a metal plate as part of the housing conductive part 112 immediately below the printed circuit board 111. When the printed circuit board 111 is connected at both ends thereof with the housing conductive part 112, the standing wave appears at a frequency where the length of board is equal to a multiple of a half wave, and this standing wave is the cause of very large radiant noise.
In the case wherein the housing conductive part is located immediately below the printed circuit board and wherein the ground portion of the printed circuit board and the housing conductive part are connected with high impedance, connection between them decreases the radiant noise at most of frequencies. In this case, however, the electric current flowing on the circuit board also generates the standing wave at a specific frequency determined by the size and dielectric of the circuit board, and it is also the cause of the radiant noise.
The conventional methods for suppressing the standing wave include a method for connecting them at plural positions and thus decreasing the intervals of connection between the connecting positions, and a method for providing the connecting portion with resistive nature as described in Japanese Laid-open Patent Application No. 7-225634.
The former method of multipoint connection is a method for changing the apparent size of the circuit board to shorten the wavelength of the standing wave generated, thereby changing the frequency into the high frequency range. The latter method for providing the connecting portion with resistive nature is a method for suppressing the electric current flowing in the connecting portion and matching the impedance of the circuit board with that of the housing conductive part to eliminate reflection, thereby suppressing the standing wave.
The conventional examples described above, however, had the following problems. The method of multipoint connection requires the increased number of components and the increased area for connection, and there are some cases where a large space cannot be used for connection to the housing from the restriction of wiring. Further, low-frequency current or DC current is likely to flow into the connecting portion from the outside, thus posing the problem of immunity.
When the connecting portion is provided with the resistive nature, the impedance must be a certain fixed value at the frequency where the standing wave appears, for suppressing the increase of noise due to the standing wave phenomenon. In that case, the impedance of the connecting portion is also the fixed value at the frequencies except for that of the standing wave, and there are some cases where suppression of current does not occur well in all of the frequency bands necessitating the countermeasures against the radiant noise. In that case there is such a possibility that the ground current undesirably flows into the connecting portion so as to be the cause of the radiant noise.
Further, when the connecting portion is provided with the resistive nature, the increase of noise due to the standing wave can be prevented, while it is difficult to adjust the impedance of the connecting portion according to change in frequency, because the fixed value is selected for the resistance from the need for impedance matching. In ordinary printed circuit boards, matching values are approximately several ten .OMEGA. in many cases, and in some cases the suppression of noise is not effected in the frequency bands necessitating the countermeasures against the radiant noise.
Particularly, for example, in the low frequency range of 300 MHz or less, a good result is obtained by connection with higher impedance than the matching value; for example, in the high frequency range of 700 MHz or more, better radiant noise characteristics are shown by connection with lower impedance than the matching value.
When the ground of the circuit board is connected with the conductive part of the housing through a series configuration of R, L, and C, actual components are affected by parasitic capacitance or parasitic inductance at the frequencies of the several hundred MHz order, and it is thus difficult to ideally change the frequency characteristics of the impedance of LC series circuit. This results in making it difficult to achieve the impedance matching at the frequency of standing wave and to avoid the increase of radiant noise due to resonance.